HVAC System Components
Automotive A/C is a refrigerant loop, and the ASE A7 test rewards reading system pressures back to a cause instead of just adding refrigerant. This reference covers the compressor, condenser, evaporator, the metering device, and the pressure controls — what each does, how it fails, and what high- and low-side gauge readings tell you. Refrigerant handling is regulated, so the test also expects correct recovery and recharge practice, not topping off a leak.
What the A/C loop does
A/C moves heat, it doesn’t make cold. The compressor pressurizes refrigerant vapor; the condenser sheds that heat and turns it to liquid; the metering device drops the pressure; and the evaporator lets the refrigerant boil and absorb cabin heat before it returns to the compressor. Heating uses engine coolant through a heater core. A7 questions hand you gauge pressures or a symptom and ask which part of the loop is at fault — and expect proper refrigerant handling throughout.
Component reference
| Component | Job | Common failure | Symptom / gauge clue |
|---|---|---|---|
| Compressor | Pressurize refrigerant | Clutch won’t engage; internal wear | No cooling; both sides equalize if not running |
| Condenser | Reject heat, condense vapor | Airflow restriction, blockage | High side spikes, poor cooling |
| Evaporator | Absorb cabin heat | Leak, freeze-up, restriction | Weak cooling; low side off |
| Metering device | Drop pressure into evaporator | Stuck/clogged | Abnormal high/low split |
| Receiver-drier / accumulator | Store refrigerant, remove moisture | Saturated desiccant, restriction | Moisture in system, variable cooling |
| Pressure switches | Cycle clutch, protect system | Faulty switch | Clutch won’t engage despite charge |
Reading the gauges
The A7 skill is pressure interpretation, not “add until cold”:
- Both sides high → overcharge or condenser airflow restriction.
- Both sides low → undercharge or a leak.
- High side high, low side low → a restriction at the metering device or drier.
- Clutch won’t engage but charge is fine → trace the control circuit and pressure switches, not the refrigerant.
Gauge patterns vs. blend-door faults
Two A7 mix-ups that cost techs points:
Refrigerant/gauge problem
- Both sides low → undercharge or leak
- Both sides high → overcharge or condenser airflow blockage
- High side high, low side low → metering device or drier restriction
- Repair: recover, fix the leak, evacuate, recharge to spec
Blend-door actuator fault
- Gauge pressures are normal
- Air-side symptom: wrong temperature zone, one vent cold and another warm
- No refrigerant-circuit explanation for the complaint
- Repair: verify actuator calibration or replace the actuator — not an A/C recharge
The call: Check gauges first. Normal pressures with a temperature-distribution complaint point to the air side (blend door or actuator), not refrigerant. EPA Section 609 governs MVAC refrigerant handling regardless of which path you take.
The parts techs mix up
- “Add refrigerant” vs. find the leak. Weak cooling with both sides low is an undercharge — but topping off a leaking system is the trap. Recover, repair, evacuate, and recharge to spec.
- Compressor vs. its control circuit. A clutch that won’t engage is often an electrical/pressure-switch problem, not a dead compressor — overlaps with A6 electrical.
- Expansion-valve vs. orifice-tube systems. They use different storage components (receiver-drier vs. accumulator) and diagnose differently; identify which the vehicle uses.
Where techs lose points
| Symptom | Real cause | Exam trap / correct call |
|---|---|---|
| No cooling, both gauge sides equalize at rest | Compressor not pumping (clutch or internal) | Check clutch engagement and control circuit before condemning the compressor |
| High side spikes, poor cooling | Condenser airflow restriction | Not an overcharge — verify airflow across the condenser first |
| Clutch won't engage, charge is normal | Control-circuit or pressure-switch fault | Trace the electrical circuit; don't assume a refrigerant problem |
| Weak cooling, both sides low | Undercharge from a leak | Topping off a leaking system is the wrong move — recover, repair, evacuate, recharge |
| Normal gauges, one side of cabin wrong temp | Blend-door actuator fault | Gauge pressures rule out refrigerant; the problem is on the air side |
ASE traps
- A/C moves heat, it doesn’t create cold — diagnose the loop, don’t just add charge.
- Refrigerant handling is regulated: EPA Section 609 governs MVAC systems — recover and recharge to spec; don’t vent or top off a known leak.
- A clutch that won’t engage with a full charge is usually a control-circuit or pressure-switch fault, not the compressor.
See electrical & HVAC study for how A7 pairs with A6, and the A6 Electrical guide for the control-circuit side of clutch and blower faults.